A flexographic printing machine of that kind is known, e.g., from Published German Application 34 37 216, and in that known machine the central gear is permanently fixed to the shaft of the impression cylinder so that the plate cylinders must have peripheral length which is an integral multiple of the tooth pitch of the central gear, which is used to drive the impression cylinder, and the diameter of the plate cylinders must be correspondingly selected. This is so because the plate cylinders and the impression cylinder will not rotate at the same peripheral velocity unless the diameters of the plate cylinders employed are exactly equal to an integral multiple of the tooth pitch and the pitch diameters of the plate cylinder gears and of the central gear match the diameters of the plate cylinders and of the impression cylinder, respectively, so that a relative velocity between the plate cylinder and the impression cylinder will be avoided and there will be no slippage or smearing of the web to be printed. But it is often necessary to print on a format having a length which requires the use of plate cylinders having a diameter which is not exactly equal to an integral multiple of the tooth pitch. In the performance of such printing jobs a certain slippage of the web to be printed on the impression cylinder and possibly also smeared print was inevitable.
It is known from Published German Application 32 39 114 that the peripheral velocity of a plate cylinder which is movable into engagement with an impression cylinder can be matched to the peripheral velocity of the impression cylinder without an occurrence of relative movements between said two cylinders. The peripheral velocities are measured and depending upon any differential peripheral velocity, velocity control means matches the peripheral velocities of the two cylinders such that relative peripheral velocity between the two rollers will be eliminated. By means of such velocity control means it is possible in a printing press to avoid, e.g., an occurrence of a relative peripheral velocity which may be the result of a thermal expansion of the cylinders.
In order to avoid an occurrence of relative movements which may occur between an impression cylinder and a plate cylinder as a result of tooth pitch difference it is known from Published German Application 34 32 572 that the impression cylinder and the associated inking roller can be driven by exactly controllable drive motors. In that press the plate cylinder and the inking roller are not operatively interconnected by gearing. But such printing press is relatively expensive because it does not have a central drive but all cylinders and rollers must be driven by separate drive motors, which must exactly be controlled.